Heat Transfer (Unit-7) Heat Exchangers IIT lecture videos

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Heat and Mass Transfer

Prof. S. P. Sukhatme

Department of Mechanical Engineering

Indian Institute of Technology, Bombay

Lecture No. 25

Heat Exchangers-1

So far we have studied the modes of heat transfer; we have studied heat conduction in solids, we have studied thermal radiation and we have also studied convection. In convection, we have looked at forced convection and natural convection. Now today we start with the topic of heat exchangers. The first thing we ask ourselves is, the first thing we will do rather is to describe various types of heat exchangers. Before that, let me just restate again the textbooks and the reference books which we are following.

Lecture No. 26

Heat Exchangers-2

Now in today’s lecture we begin with the topic of mean temperature difference in a heat exchanger. We are going to first define the term - what we mean by the mean temperature difference in a heat exchanger - and then we are going to derive expressions for the mean temperature difference for various flow classifications, that is for parallel flow, counter flow, the 3 cases of cross flow and so on. Let us consider some heat transfer surface.

Lecture No. 27

Heat Exchangers – 3

We defined the term called mean temperature difference and we derived expressions for the mean temperature difference in parallel flow and in counter flow. You would recall that the logarithm of delta Ti delta Te occurs in the denominator of the expressions, both the expressions. For this reason the mean temperature difference in a heat exchanger is also referred to as the logarithmic mean temperature difference and the symbol capital LMTD - all letters in capital - LMTD is used instead of delta Tm by many authors and many writers.

Lecture No. 28

Heat Exchanger – 4

So, we have discussed the mean temperature difference method for analyzing heat exchangers. We derived expressions for the mean temperature difference in various flow arrangements, for various flow arrangements like parallel flow, counter flow, all the 3 cases of cross flow. And once we had those expressions or the results in the form of graphs, we were able to show through numerical examples how one does calculations using the basic performance equation q is equal to UA delta Tm. Now today, we will look another approach, discuss a method called the effectiveness NTU method.